The concept of Fiber Reinforced Elastomeric Isolators (FREIs) was initially proposed to reduce the high cost and weight associated with conventional Steel Reinforced Elastomeric Isolators (SREIs) (Kelly, 1999). SREIs are bearings comprising a laminate structure of alternating layers of elastomeric material and steel sheet reinforcement, with the external layers being of elastomeric material. FREI bearings are similar to SREI bearings, except that the reinforcement takes the form of fiber rather than steel sheet. Thus, the laminate structure of an SREI bearing comprises a plurality of layers of elastomeric material, two of which form external layers of the laminate structure and define opposed contact surfaces of the isolator body, and a plurality of layers of fiber reinforcement. Each layer of fiber reinforcement is disposed between a pair of adjacent layers of elastomeric material, and each pair of adjacent layers of elastomeric material is bonded together across the layer of fiber reinforcement disposed therebetween. Construction of SREI bearings is known in the art, and is described, for example, in U.S. Pat. No. 8,291,651 to Drysdale et al., the teachings of which are hereby incorporated by reference in their entirety.
Experimental testing of FREIs revealed that similar performance to SREIs could be obtained with the benefit of additional damping attributed to the inter-fiber movement of the reinforcement (Kelly 1999).
In addition to replacing the steel reinforcement sheets, the design and installation of FREIs can be further simplified by installing the isolator body unbonded to the supports (i.e. the superstructure and substructure), as taught by U.S. Pat. No. 8,291,651 to Drysdale et al. Drysdale et al. teach FREI bearings that are located between the superstructure and substructure of a building with no bonding at the contact surfaces of the isolator body, which they refer to as an “unbonded application”. In such an application, FREI bearings, with the appropriate aspect ratio, exhibit stable rollover deformation.
Reference is now made to FIGS. 1(a) to 1(d) and FIGS. 1(e) to 1(h), which show the behavior of a FREI bearing 100 comprising an isolator body 102 in an unbonded application at different amplitudes of lateral displacement. FIGS. 1(a) to 1(d) show displacement of the upper support 104 to the right and FIGS. 1(e) to 1(h) show displacement of the upper support 104 to the left; the vertical lines 103 are provided to show relative displacement. FIGS. 1(a) and 1(e) show displacement of 0% tr, FIGS. 1(b) and 1(f) show displacement of 100% tr, FIGS. 1(c) and 1(g) show displacement of 150% tr and FIGS. 1(d) and 1(h) show displacement of 200% tr, where tr is the total thickness of the elastomeric layers. As can be seen in FIGS. 1(a) to 1(d) and FIGS. 1(e) to 1(h), in a FREI bearing utilized in unbonded application, as the supports 104 are displaced laterally relative to one another, the contact surfaces 112 (top and bottom faces) of the FREI bearing 100 roll off the upper and lower contact supports 104, respectively. This lateral deformation is described as “rollover deformation”. It occurs as a result of both unbonded application and the lack of flexural rigidity of the fiber reinforcement sheets. A properly designed FREI bearing can sustain very large lateral displacements while remaining stable, by maintaining a positive incremental lateral load resisting-capacity throughout the hysteresis loops. As such, the resulting permissible deformation is called “stable rollover” (SR) deformation. At the extreme lateral displacements shown in FIGS. 1(d) and 1(h), the free edges 116 engage the upper and lower contact supports 104, inhibiting further lateral displacement and resulting in an observable hardening, which limits the maximum lateral displacement of the FREI bearing 100 and enhances its overall stability.
SR-deformation results in a significant decrease in the horizontal stiffness of the bearing and adds to its efficiency as a seismic isolator. An unbonded FREI bearing that exhibits SR-deformation is described by Drysdale et al. as a “stable unbonded” (SU) FREI (note that Drysdale et al. contains the typographical error “unbounded”). In a SU-FREI bearing, the “full contact vertical face lateral displacement” (δfc) occurs when the free edges 116 (which, without horizontal displacement of the supports, are the original vertical faces of the isolator body 102) completely engage the horizontal upper and lower supports 104, as shown in FIGS. 1(d) and 1(h). As a result, significant stiffening in the hysteresis loops is observed which effectively places a limit on the extreme lateral displacements that can occur under unanticipated seismic excitation levels. Rollout instability lateral displacement, denoted as δmax, for this particular type of bearing is significantly larger than δfc.
Placing the isolator body unbonded between the supports prevents the development of moment in the isolator body, thus eliminating the high tensile stress regions that usually develop as the isolator body is displaced horizontally reducing the tensile stress demand on the elastomer (Toopchi-Nezhad et al. 2011). Unlike rigid steel reinforcement, fiber reinforcement is assumed to be extensible and to provide no appreciable resistance in bending. The lack of bending rigidity, combined with the unbonded installation, results in a unique rollover deformation in the end regions that would otherwise be in tension as illustrated in FIG. 2(a) and FIG. 2(b). In FIGS. 2(a) and 2(b), corresponding reference numerals are used for features corresponding to features of the FREI 100 in FIG. 1 except with the prefix “2” instead of “1” and the suffix “a” or “b”, respectively. FIG. 2(a) shows the behavior of a SU-FREI 200a, and FIG. 2(b) shows the behavior of a FREI 200b whose contact surfaces 212b have been fully bonded to the supports 204b. The SU-FREI 200a undergoes rollover deformation in the rollover regions 240a adjacent the free edges 216a whereas the FREI 200b whose contact surfaces 212b have been fully bonded to the supports 204b is subjected to tension in the regions 240b along the free edges 216b. 
As the horizontal displacement increases and the amount of the isolator contact surface that loses contact with the supports increases, the effective horizontal stiffness decreases. Toopchi-Nezhad et al. (2008) demonstrated experimentally that the reduction in effective horizontal stiffness would result in instability as the tangential stiffness becomes negative. It was determined that by increasing the aspect ratio, defined as the ratio of the width in direction of horizontal displacement (displacement axis of the isolator body) to the total height of the isolator (measured along the free edges of the isolator body), instability could be reduced. In computing the aspect ratio of a FREI, height and width are measured in the unstressed condition (without tension or compression). The increase in horizontal stiffness occurred as the free edges of the isolator contacted the upper and lower supports, completing full rollover. The stiffening of the isolator was identified as an advantageous characteristic both to protect against instability and to prevent excessive displacements during beyond design basis events (Toopchi-Nezhad et al. 2008).
A design limitation associated with unbonded FREIs originates from the unbonded application, which is also responsible for the desirable softening and stiffening characteristics. The unbonded application prevents the transfer of tensile forces through the isolator, making this type of isolator unsuitable for situations where a tensile vertical design load must be resisted. A tensile vertical design load may occur in near fault applications or in situations where overturning is of concern. A vertical design load can occur either as a result of the geometry of the structure or due to the location of the structure in a geographical region with a high expected vertical acceleration component. Provisions for tensile testing are often provided and required in design codes and standards (ISO 2010, ASCE 2010). In general, experimental testing of elastomeric isolators subjected to tension is limited due to difficulties in simultaneously applying a tensile load while displacing the isolator horizontally (Naeim and Kelly, 1999). Theoretical analysis of SREIs has identified that the isolator may also buckle in tension (Kelly and Konstantinidis, 2011). Therefore as the tensile load increases, the horizontal stiffness of a bonded SREI will decrease.
In addition, residual displacement can occur in unbonded FREIs as a result of slip at the interface between the isolator body and the supports if the frictional resistance along the contact surfaces is exceeded. An experimental investigation and literature review on the neoprene-concrete friction relationships was conducted by Magliulo et al. (2011). A comparison among code equations, results from existing studies in the literature, and the experimental data presented in the study demonstrated substantial variation in the friction coefficient as a function of vertical compressive stress. Magliulo et al. (2011) contributed the variation in part to differences in the roughness of the concrete surface. It was stated that in many of the reviewed studies the quality and finish of the concrete surface was not specified or discussed. This variation contributes to uncertainties of the horizontal force transfer capacity of unbonded FREIs.
While conducting an experimental shake table program on a scaled base isolated structure with unbonded FREIs, Foster (2011) identified that slip occurred in a select scaled earthquake record where the seismic demand significantly exceeded the design basis of the isolation system. The resulting residual displacement was observed when the peak displacement of the isolation layer reached 3.11 tr, where tr is the total thickness of the elastomeric layers. This peak displacement was far in excess of the full rollover displacement of approximately 2.00 tr for the isolators considered in the study. The level of residual displacement was reduced by 66% when grit paper was introduced to increase the friction between the elastomer and the otherwise steel and aluminum support surfaces. The introduction of grit paper was found to have negligible influence on the response of the structure, but resulted in a substantial decrease in the level of residual displacement. This study demonstrates the importance of frictional properties on the performance of the isolation system in preventing slip at large peak displacements that may occur in beyond design basis events.
Russo and Pauletta (2013) experimentally investigated the friction properties of unbonded FREIs on concrete surfaces with varying vertical compressive stress. In the experimental program it was observed that at the end of a single horizontal displacement cycle that the contact surface of the isolator had some level of residual displacement. The magnitude of the residual displacement was shown to be a function of the vertical compressive stress and the location along the surface of the isolator attributed to the rollover deformation. Moment equilibrium in unbonded FREIs is maintained by a change in the vertical stress distribution; areas of the contact surface with high vertical stress concentrations displayed less residual deformation than areas with lower vertical stress.
From these studies it can be concluded that while the friction properties of unbonded FREIs is an area that necessitates further investigation, another design limitation affecting unbonded FREIs is the potential for residual displacement resulting from slip at the interface between the isolator body and the supports.
Thus, while unbonded FREIs provide the benefit of stable rollover deformation, they do not permit the transfer of tensile forces through the isolator and are subject to residual displacement.